System and method for establishing an xdsl data transfer link

ABSTRACT

An xDSL data transfer system for data transfer comprising at least one xDSL user modem ( 4 ) connected via a data transfer medium ( 8 ) to a corresponding xDSL modem ( 9 ) within a central office, wherein the xDSL user modem ( 4 ) generates a pulse length modulated wake-up signal for switching the corresponding xDSL modem ( 9 ) within the central office ( 10 ) from a sleep mode to an operation mode.

[0001] The invention relates to a system and a method for establishing adata transfer link between an xDSL user modem and a corresponding xDSLmodem within a central office.

[0002]FIG. 1 shows a conventional xDSL system with several xDSL usermodems, each connected via a telephone line to a corresponding xDSLmodem within the central office. In a normal operation, the conventionalxDSL modem within the central office is operating continuously and triesto establish a data transfer link with the corresponding remote userxDSL modem connected via a telephone line. When the user xDSL modem isready for data transfer, a data link is established by means of astart-up procedure. A central office comprises a very large number ofxDSL modems connecting the users of the telephone network. Thedisadvantage of the conventional xDSL system as shown in FIG. 1 is thatthe xDSL modems within the central office continuously try to establisha connection with the corresponding user xDSL modem and thereforeconsume much power. An xDSL modem within a central office consumes up to1,5 Watt of power. Since the number of xDSL modems within a centraloffice can be very large, the power consumption of all xDSL modems issignificant.

[0003] Accordingly, it is the object of the present invention to providea method and a system for data transfer between an xDSL user modem and acorresponding xDSL modem within a central office which consume a minimumamount of power.

[0004] This object is achieved by a method for establishing a datatransfer link between an xDSL user modem and a corresponding xDSL modemwithin a central office having the features of claim 1 and by an xDSLdata transfer system having the features of patent claim 8.

[0005] The present invention provides a method for establishing a datatransfer link between an xDSL user modem and the corresponding xDSLmodem within the central office comprising the following steps:

[0006] generating a wake-up bit pattern identifying the user modem,pulse length modulating the generated wake-up bit pattern to generate apulse length modulated wake-up signal,

[0007] transmitting the pulse length modulated wake-up signal from theuser xDSL modem via a data transfer medium to the corresponding xDSLmodem within the central office,

[0008] demodulating the transmitted wake-up signal by the xDSL modemwithin the central office,

[0009] comparing the demodulated wake-up signal with a stored wakeup bitpattern for the detection of a transmission of the wake-up bit pattern,

[0010] generating a wake-up command signal for switching the xDSL modemwithin the central office from a sleep mode to an operation mode fordata transfer, when the wake-up bit pattern is detected.

[0011] In the sleep mode, the power consumption of the xDSL modem withinthe central office is low, thus minimizing the overall power consumptionwithin the central office.

[0012] In a preferred embodiment of the method according to the presentinvention, the xDSL modem within the central office commences a start-upprocedure, when it is switched to the operation mode.

[0013] In a further embodiment, the wake-up signal is transmittedperiodically by the xDSL user modem.

[0014] In a further embodiment, the xDSL modem within the central officeis switched from the operation mode to the sleep mode when the datatransfer is finished.

[0015] In a further preferred embodiment, a detection counter isincremented when the wake-up bit pattern is detected.

[0016] In a further preferred embodiment, a wake-up command signal isgenerated when the detection counter reaches a predetermined thresholdvalue.

[0017] In a preferred embodiment, the threshold value for the detectioncounter is adjusted.

[0018] The invention provides further an xDSL data transfer system fordata transfer comprising at least one xDSL user modem connected via adata transfer medium to a corresponding xDSL modem within the centraloffice,

[0019] wherein the xDSL user modem generates a wake-up signal forswitching the corresponding the xDSL modem within the central officefrom a sleep mode to an operation mode.

[0020] In a preferred embodiment, the wake-up signal is pulse-lengthmodulated.

[0021] In a preferred further embodiment, the xDSL user modem comprises

[0022] generating means for generating a wake-up pattern identifying thexDSL user modem, and

[0023] modulating means for pulse length modulation of the wake-uppattern to generate a pulse length modulation wake-up signal, whereinthe pulse length modulated wake-up signal has a spectrum within the xDSLupstream frequency band.

[0024] In a preferred embodiment of the xDSL data transfer systemaccording to the present invention, the generated wake-up bit patterncomprises 16 bits.

[0025] In a further embodiment, each bit of the wake-up bit patterndetermines the duration of a pulse in the pulse length modulated signal.

[0026] In a still further embodiment, the level of the wake-up signal isless than a predetermined maximum PSD-signal level.

[0027] In a further embodiment of the xDSL data transfer systemaccording to the present invention, the xDSL modem within the centraloffice comprises

[0028] demodulating means for demodulating the received analog pulselength modulated signal,

[0029] storing means for storing a wake-up bit pattern which identifiesa user modem,

[0030] detecting means for detecting the demodulated wake-up bit patternby comparing the demodulated received signal with the stored wake-up bitpattern,

[0031] wake-up command generating means for generating a wake-up commandto switch the xDSL modem from the sleep mode to the operation mode, whenthe received wake-up bit pattern is identical with the stored wake-upbit pattern.

[0032] In a further embodiment of the xDSL data transfer systemaccording to the present invention, the demodulating means of the xDSLmodem comprises a gain sequencer for amplifying the received analogsignal with an adjustable gain, rectifying means for rectifying theamplified signal, a low-pass filter for filtering the rectified signal,and a comparator for comparing the filtered signal with an adjustablethreshold generating an asynchronous pulse train supplied to thedetecting means.

[0033] In a preferred embodiment of the xDSL data transfer systemaccording to the present invention, the detecting means comprises

[0034] a synchronization means for synchronizing the asynchronous pulsetrain with an internal clock signal,

[0035] a pulse length detecting circuit for detecting the pulse lengthof each pulse in the synchronized pulse train,

[0036] a bit pattern generating means for generating a bit patterndepending on the detected pulse length,

[0037] a register for temporarily storing the received bit pattern,

[0038] a comparator which compares the received bit pattern with thestored wake-up bit pattern and which increments a counter when thereceived bit pattern and the stored wake-up bit pattern is identical.

[0039] In a preferred embodiment of the xDSL system according to thepresent invention, the wake-up command generating means generates thewake-up command when the counter reaches a predetermined thresholdvalue.

[0040] In a preferred embodiment of the xDSL system according to thepresent invention the xDSL modems are VDSL modems.

[0041] In a preferred embodiment of the xDSL data transfer system, thedata transfer medium is a telephone line.

[0042] The invention provides further an xDSL user modem comprising

[0043] a generating means for generating a wake-up pattern identifyingthe xDSL user modem, and

[0044] a modulating means for pulse length modulation of the wake-up bitpattern to generate the pulse length modulated wake-up signal,

[0045] wherein the pulse length modulated wake-up signal has a spectrumwithin the xDSL upstream frequency band.

[0046] The invention provides further an xDSL central office modemwithin a central office comprising

[0047] demodulating means for the demodulation of a received analogpulse length modulated signal,

[0048] storing means for storing a wake-up bit pattern which identifiesthe user modem connected to the xDSL office modem via a data transfermedium,

[0049] detecting means for detecting a demodulated wake-up bit patternby comparing the demodulated received signal with the stored wake-up bitpattern,

[0050] a wake-up command generating means for generating a wake-upcommand to switch the xDSL modem from the sleep mode to the operationmode when the received wake-up bit pattern is identical with the storedwake-up pattern.

[0051] In the following description, a preferred embodiment of a methodfor establishing a data transfer link and an xDSL data transfer systemfor data transfer according to the present invention is described withreference to the drawings.

[0052]FIG. 1 shows an xDSL data transfer system for data transferaccording to the state of the art;

[0053]FIG. 2 shows an xDSL data transfer system for data transferaccording to the present invention;

[0054]FIG. 3 shows a block diagram of additional units within an xDSLuser modem according to the present invention;

[0055]FIG. 4 shows a block diagram of an xDSL modem within the centraloffice according to the present invention;

[0056]FIG. 5 shows demodulating means of an xDSL modem within thecentral office according to the present invention;

[0057]FIG. 6 shows a block diagram of detecting means of the xDSL modemwithin the central office according the present invention;

[0058]FIG. 7 shows a timing diagram of the pulse length modulatedwake-up signal according to the present invention;

[0059]FIG. 8a, b show a flow diagram of the generation of a wake-upcommand signal by an xDSL modem within the central office according tothe present invention.

[0060] The xDSL data transfer system 1 for data transfer according tothe present invention is shown in FIG. 2. The xDSL data transfer systemcomprises N data sources 2-1 to 2-N, each generating a data upstreamsignal. The data upstream signal is supplied via signal lines 3-1 to 3-Nto the corresponding xDSL user modem 4-1 to 4-N. The xDSL user modem 4consists of a conventional xDSL modem 5 which is connected on its outputside via a line 6 to a wake-up signal generating unit 7 which generatesa wake-up signal. The xDSL user modem 4 is connected via a physicalmedium, e.g. a twisted telephone line, to a corresponding xDSL modem 9within a central office 10. Each xDSL modem 9 of the central office 10comprises a conventional xDSL modem 11 and a wake-up signal detectionunit 12 for detecting a wake-up signal received via the telephone line8. The wake-up signal detection unit 12 is connected to the telephone 8via line 13 and sends a wake-up command signal via a control line 14 tothe xDSL modem 11 to switch the xDSL modem from a sleep mode to anoperation mode for data reception.

[0061] The wake-up signal generating unit 7 of the xDSL user modem 4generates a wake-up signal via the telephone line 8 to its correspondingxDSL modem 9 within the central office 10. The transmitted wake-upsignal is detected by the wake-up signal detection unit 12 so that thexDSL modem 11 is switched from a sleep mode to an operation mode fordata transfer. Each user modem 4 generates a wake-up signal which isunique to the user modem 4 so that the wake-up signal detection unit 12is insensitive to far end cross-talk (FEXT) between different telephonelines 8. The unique wake-up signal for each user modem 4 guarantees thaterroneous wake-ups of other xDSL-modems as a consequence of far endcross-talk (FEXT) are prevented. Accordingly, the different telephonelines 8-1 to 8-N can be arranged in a common telephone cable 15.

[0062] When an xDSL user modem 4 is inactive, the corresponding xDSLmodem 9 within the central office 10 is in a sleep mode. Since a datasource 2 like a personal computer 2 normally does not send a dataupstream signal to the central office for the most part of the day, thecorresponding xDSL modem 9 within the central office 10 is in the sleepmode for most of the time. In the sleep mode, the xDSL modem 9 consumesonly a minimum necessary amount of electrical power. The xDSL modem 9 isswitched to the power-consuming operation mode only when the wake-upsignal from the corresponding xDSL user modem 4 is detected. When theuser turns on the power of the user modem 4 and wishes to establish adata connection, the user modem 4 generates a wake-up signal and sendsit via the telephone line 8 to the corresponding xDSL modem 9 in thecentral office. The wake-up signal detection unit 12 detects a receivedwake-up signal-and switches the xDSL modem 9 to an operation mode. ThexDSL modem 9 then commences a start-up procedure. When the user ends thedata connection, the xDSL modem 9 within the central office 10 isswitched back to the sleep mode.

[0063]FIG. 3 shows a wake-up signal generating unit 7 within an xDSLuser modem 4 according to the present invention. The wake-up signalgenerating unit 7 comprises a wake-up pattern generating means 16 forgenerating a wake-up bit pattern identifying the xDSL user modem 4. Thegenerated wake-up bit pattern is supplied via a line 17 to a modulationmeans 18. The modulation means 18 performs a pulse length modulation ofthe upstream data signal supplied from the data source 2 via data line6. The upstream data signal is pulse length modulated with the wake-upbit pattern to generate a pulse length modulated wake-up signal which istransmitted via line 8 to the corresponding xDSL modem 9 within thecentral office 10. The pulse length modulated wake-up signal has afrequency range within the xDSL upstream frequency band. For a VDSLapplication, the upstream frequency band is between 3,5 MHz and 5 MHz.The generated wake-up signal can pass through passive magnetic elementssuch as transformers, and does not get blocked before it reaches thewake-up detection unit 12 within the xDSL modem 9 in the central office10.

[0064] In a preferred embodiment, the wake-up bit pattern generatingmeans 16 generates a wake-up bit pattern which comprises 16 bits. Thiscomparatively short bit pattern ensures that the wake-up process iscompleted within a relatively short time, e.g. within 100 milliseconds.

[0065]FIG. 7 shows an example for a wake-up signal generated by thewake-up generating means 7 of a user xDSL modem 4. The wakeup signal hasa flat frequency spectrum in the xDSL upstream frequency band modulatedby a duty cycle modulation. A sequence of N bits each of a duration of dmicroseconds is transmitted periodically by the user modem 4 until it isdetected by the wake-up detection unit 12 within the xDSL modem 9 of thecentral office 10. Each bit of the wake-up bit pattern is represented bya pulse of d_(on) microseconds on and d_(off) microseconds off, whered_(on)<d_(off), if the bit is logical low, and d_(on)>d_(off), if thebit is logical high. In a preferred embodiment, the wake-up bit patterncomprises 16 bits, duration d is approximately 100 microseconds, whereind_(on)=0,25 d, when the bit is logical low, and d_(on)=0,75 d, when thebit is logically high. FIG. 7 shows an example for a wake-up signalduring one period of the wake-up signal. FIG. 7 shows only the signalenvelope. The signal itself is a random noise signal whose spectrumcoincides with the spectrum of a standard xDSL upstream signal.

[0066]FIG. 4 shows a block diagram of the wake-up signal detection unit12 within an xDSL modem 9 of the central office 10. The wake-up signaldetection unit 12 comprises demodulating means 19 for demodulating thereceived analog pulse length modulated signal. The demodulating means 19are connected via a line 20 to comparing means 21 which compare thedemodulated received bit pattern with the wake-up bit pattern storedwithin storing means 22. The storing means 22 store a wake-up bitpattern which identifies the corresponding user modem 4. The storingmeans 22 are connected to the comparing means 21 via a line 23. If thecomparator 22 detects that the demodulated bit pattern is identical withthe stored wake-up bit pattern, it sends a detection signal via a line24 to a wake-up command generating means 25 which generates the wakeupcommand and transmits the wake-up command via a control line 14 to thecorresponding modem 11. The wake-up command switches the xDSL modem 9from the sleep mode to the operation mode. The wake-up signal detectionunit 12 can be implemented by a simple circuitry that does not requirelarge power. Accordingly, the power consumption of the wake-up signaldetection unit 12 is only a small percentage of the power consumed bythe complete xDSL modem 9. The wake-up signal detection unit isinsensitive to the absolute power level of the received wake-up signalin order to accommodate all line distances. The wake-up signal detectionunit 12 is provided for detecting a bit pattern, not a signal energy.

[0067]FIG. 5 shows a preferred embodiment of the demodulating means 19within the wake-up detection unit 12 according to the present invention.The demodulating means 19 comprises a gain sequencer 26 for amplifyingthe received analog signal with an adjustable gain, the rectifyingcircuit 27 rectifying the amplified signal, a low-pass filter 28 forfiltering the rectified signal and a comparator 29 for comparing thefiltered signal with an adjustable threshold generating an asynchronouspulse train supplied to the detecting means 21. The gain sequencer 26amplifies the incoming wake-up signal and comprises an operationamplifier 30, resistors 31 a, 31 b and adjustable resistors 32 a, 32 b.The gain sequencer 26 amplifies the incoming wake-up signal with anincreasing gain until the wake-up signal is detected by the wake-updetection means. The rectifying means 27 comprising an operationalcontrol amplifier 33 which controls switches 34 a, 34 b supplies arectified signal to the following low-pass filter 28. The low-passfilter 28 shown in FIG. 5 removes the high frequency noise and tracksthe signal envelope of the received wake-up signal. The low-pass filter28 further removes residual DC signal components. The low-pass filter 28is an active filter comprising an operational amplifier 35, resistors36, 37, 38 and capacitors 39, 40. The output of the operationalamplifier 35 is connected via line 41 to comparing means 29 including acomparator 42, a resistor 43 a and a capacitor 44 a.

[0068] The output of the comparator 42 is connected via line 20 to thebit pattern comparing means 21 comparing the output bit pattern of thecomparator 42 with the stored wake-up bit pattern. The bit patterncomparing means 21 generates a detecting signal with the received bitpattern output by a comparator 42 which is identical with the wake-upbit pattern stored in the storage means 22 and sends the detectingsignal to the wake-up command generating means 25.

[0069]FIG. 6 shows a preferred embodiment of the bit pattern detectingmeans 21. The bit pattern detecting means 21 comprises a synchronizationcircuit 43 for synchronizing the asynchronous pulse train received fromthe demodulating means 19. The received pulse train or bit pattern issynchronized with an internal clock signal. The detecting means 21further comprises a pulse length detecting circuit 44 connected to thesynchronization circuit 43 via line 45. The output of the pulse lengthdetecting circuit 44 is connected on its output side via line 46 to aregister 47 for temporarily storing the received bit pattern. A bitcomparator 48 connected to the register 47 via a line 49 compares thereceived bit pattern stored in the register 47 with the wake-up bitpattern stored in the memory 22 and increments a counter 51 via acontrol line 50, when the received bit pattern and the stored bitpattern is identical. The wake-up command generating means 25 generatesa wake-up command, when the counter 51 reaches an adjustable thresholdvalue.

[0070]FIG. 8a shows a flow diagram of a method for establishing a datatransfer link between the xDSL user modem 4 and the corresponding xDSLmodem 9 in the central office 10 according to the present invention.

[0071] In a step S1, a wake-up bit pattern identifying the xDSL usermodem 4 is generated by a bit pattern generating circuit 16.

[0072] In a step S2, the generated wake-up bit pattern is used for pulselength modulating an upstream data signal to generate a pulse lengthmodulated wake-up signal by means of a modulating circuit 18.

[0073] The pulse length modulated wake-up signal is transmitted from theuser xDSL modem 4 via the telephone line 8 to the xDSL modem 9 withinthe central office 10 in a step S3.

[0074] In a further step S4, the transmitted wake-up signal isdemodulated by demodulating means 19 within the wake-up signal detectionunit 12 of the xDSL modem 9.

[0075] In a step S5, the demodulated wake-up signal is compared with awake-up bit pattern stored in the register 22.

[0076] In step S6, a wake-up command signal is generated when thereceived bit pattern is identical with the stored wake-up bit pattern,and the modem 11 is switched from a sleep mode to an operation mode tostart a start-up procedure.

[0077] After the modem 11 has been switched into the operation mode, thexDSL modem 9 commences a start-up procedure to establish a data linkwith the corresponding xDSL user modem 4. After the start-up procedure,the normal data transfer is performed. When the data transfer isfinished, the xDSL modem 9 within the central office 10 is switched fromthe operation mode to the sleep mode.

[0078]FIG. 8b shows a preferred embodiment of step S6 within the mainroutine according to the present invention as shown in FIG. 8a. Thereceived bit pattern stored temporarily in register 47 is compared tothe wake-up bit pattern stored in register 22, and when both bitpatterns are identicaly, counter 51 of comparing means 21 is incrementedin a step S6-2. The value of the counter 51 is compared in step S6-3with a predetermined threshold value, and a wake-up command signal isgenerated in step S6-4 when the counter reaches the threshold value. Thewake-up command signal switches the xDSL modem 9 from the sleep mode tothe operation mode. List of reference numerals 1 xDSL data transfersystem 2 data source 3 line 4 xDSL user modem 5 modem 6 line 7 wake-upsignal generating circuit 8 telephone line 9 xDSL modem 10 centraloffice 11 modem 12 wake-up signal detecting unit 13 line 14 line 15cable 16 bit pattern generating means 17 line 18 half-length modulationcircuit 19 demodulating means 20 line 21 comparing means 22 storingmeans 23 line 24 line 25 wake-up command generating means 26 trainsequencer 27 rectifying means 28 low-pass filter 29 comparing means 30operation amplifier 31 resistors 32 adjustable resistors 33 operationalamplifier 34 switches 35 operational amplifier 36 resistor 37 resistor38 resistor 39 capacitor 40 capacitor 41 line 42 operational amplifier43 resistor 44 capacitor 43 synchronization means 44 pulse lengthdetecting circuit 45 line 46 line 47 register 48 bit comparator 49 line50 line 51 counter

1. Method for establishing a data transfer link between an xDSL usermodem (4) and a corresponding xDSL modem (9) within a central office(10) comprising the following steps: (a) generating (S1) a wake-up bitpattern identifying the user xDSL modem (4); (b) pulse length modulating(S2) an upstream data signal with the generated wake-up bit pattern togenerate a pulse length modulated wake-up signal; (c) transmitting (S3)the pulse length modulated wake-up signal from the user xDSL modem (4)via a data transfer medium (8) to the xDSL modem (9) within the centraloffice (10); (d) demodulating (S4) the transmitted wake-up signal; (e)comparing (S5) the modulated wake-up signal with a stored wake-up bitpattern for the detection of a transmission of the wake-up bit patternfrom the xDSL user modem (4); (f) generating (S6) a wake-up commandsignal, when the wakeup bit pattern is detected for switching the xDSLmodem (11) within the central office (10) from a sleep mode to anoperation mode for data transfer.
 2. The method according to claim 1,wherein the xDSL modem (9) within the central office (10) commences astart-up procedure when it is switched to the operation mode.
 3. Themethod according to claim 1, wherein the wake-up signal is transmittedperiodically by the xDSL user modem (4).
 4. The method according to oneof the preceding claims, wherein the xDSL modem (9) within the centraloffice (10) is switched from the operation mode to the sleep mode whenthe data transfer is finished.
 5. The method according to one of thepreceding claims, wherein a detection counter (51) is incremented whenthe wake-up bit pattern is detected.
 6. The method according to one ofthe preceding claims, wherein the wake-up command signal is generatedwhen the detection counter (51) reaches a threshold value.
 7. The methodaccording to claim 6, wherein the threshold value is adjusted.
 8. AnxDSL data transfer system for data transfer comprising at least one xDSLuser modem (4) connected via a data transfer medium (8) to acorresponding xDSL modem (9) within a central office (10), wherein thexDSL user modem (4) generates a pulse length modulated wake-up signalfor switching the corresponding xDSL modem (9) within the central office(10) from a sleep mode to an operation mode.
 9. The xDSL data transfersystem according to claim 8, wherein the xDSL user modem (4) comprisesgenerating means (16) for generating a wake-up bit pattern identifyingthe xDSL user modem (4), and modulating means (18) for the pulse lengthmodulation of an upstream data signal with the wake-up bit pattern togenerate the pulse length modulated wake-up signal, wherein the pulselength modulated wake-up signal has a spectrum within the xDSL upstreamfrequency band.
 10. The xDSL data transfer system according to one ofthe preceding claims, wherein the generated wake-up bit patterncomprises 16 bits.
 11. The xDSL data transfer system according to one ofthe preceding claims, wherein each bit of the wake-up bit patterndetermines the duration of a pulse length of a pulse of the pulse lengthmodulated wake-up signal.
 12. The xDSL data transfer system according toone of the preceding claims, wherein the level of the wake-up signal isless than a predetermined maximum PSD-level.
 13. The xDSL data transfersystem according to one of the preceding claims, wherein the xDSL modem(9) within the central office (10) comprises: demodulating means (19)for demodulating the received analog pulse length modulated signal;storing means (22) for storing a wake-up bit pattern which identifies acorresponding xDSL user modem (4); comparing means (21) for comparingthe received demodulated signal with the stored wake-up bit pattern; andwake-up command generating means (25) for generating a wakeup command toswitch the xDSL modem (9) from the sleep mode to the operation mode,when the demodulated wake-up signal is identical with the stored wake-upbit pattern.
 14. The xDSL data transfer system according to claim 13,wherein the demodulating means (19) of the xDSL modem (9) comprises again sequencer (26) for amplifying the received analog signal with anadjustable gain; rectifying means (27) for rectifying the amplifiedsignal; a low-pass filter (28) for filtering the rectified signal; and acomparator (29) for comparing the filtered signal with an adjustablethreshold generating an asynchronous pulse train which is supplied tothe bit pattern comparing means (21).
 15. The xDSL data transfer systemaccording to one of the preceding claims, wherein the bit patterncomparing means (21) comprises a synchronization means (43) forsynchronizing the asynchronous pulse train with an internal clocksignal; a pulse length detecting circuit (44) for detecting the pulselength of each pulse in the synchronized received pulse train andgenerating a logical bit value corresponding to the detected pulselength; a register (47) for temporarily storing the received bitpattern; a comparator (48) which compares the received bit pattern withthe stored wake-up bit pattern and increments a counter (51), when thereceived bit pattern and the stored wake-up bit pattern is identical.16. The xDSL data transfer system according to one of the precedingclaims, wherein the wake-up command generating means (25) generates thewake-up command, when the counter (51) reaches an adjustable thresholdvalue.
 17. The xDSL data transfer system according to one of thepreceding claims, wherein the xDSL modems (4, 11) are VDSL modems. 18.The xDSL data transfer system according to one of the preceding claims,wherein the data transfer medium (8) is a telephone line.
 19. An xDSLuser modem (4) comprising a generating means (16) for generating awake-up bit pattern identifying the xDSL user modem (4); and modulatingmeans (18) for the pulse length modulation of an upstream xDSL datasignal with the generated wake-up bit pattern to generate a pulse lengthmodulated wake-up signal, wherein the pulse length modulated wake-upsignal has a frequency range within the xDSL upstream frequency band.20. An xDSL modem (11) within a central office (10) comprising:demodulating means (19) for demodulating a received analog pulse lengthmodulated signal; storing means (22) for storing a wake-up bit patternidentifying an xDSL user modem (4); comparing means (21) comparing thereceived demodulated signal with the stored wake-up bit pattern; wake-upcommand generating means (25) for generating a wakeup command to switchthe xDSL modem (9) from a sleep mode to an operation mode, when thereceived bit pattern is identical with the stored wake-up bit pattern.